Field of the Invention
The present invention relates to a packaging material for a lithium ion battery.
Description of the Related Art
As secondary batteries, nickel hydrogen storage batteries and lead storage batteries are known, but miniaturization of the secondary batteries is necessary due to miniaturization of portable devices, restriction on installation space, and the like, and thus lithium ion batteries having a high energy density have attracted attention. As a packaging material for a lithium ion battery (hereinafter, also simply referred to as “packaging material”) which is used in lithium ion batteries, a metal casing is used in the related art. However, recently, a multi-layer film, which is light weight, has high heat dissipation properties, and is capable of supporting the packaging material at a low cost, has been used.
An electrolytic solution of lithium ion batteries is composed of an aprotic solvent such as propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate, and an electrolyte. In addition, as a lithium salt that is the electrolyte, a lithium salt such as LiPF6 and LiBF4 is used. However, the lithium salt generates hydrofluoric acid through a hydrolysis reaction due to moisture, and thus corrosion in a metallic surface of a battery member due to the hydrofluoric acid or a reduction in lamination strength between the layers of the packaging material constituted by the multi-layer film may be caused. Therefore, the packaging material constituted by the multi-layer film is generally provided with an aluminum foil layer at the inside to suppress intrusion of moisture from an outer surface of the multi-layer film. For example, a packaging material for a lithium ion battery is known in which a base material layer having heat resistance/a first adhesive layer/an aluminum foil layer/a corrosion prevention-treated layer that prevents corrosion due to hydrofluoric acid/a second adhesive layer/a sealant layer are sequentially laminated. A lithium ion battery using the packaging material for a lithium ion battery is called an aluminum laminated lithium ion battery.
The packaging material constituted by the laminated film is largely classified into two types according to the kind of the second adhesive layer. That is, the packaging material is largely classified into a dry laminate configuration using an adhesive for dry laminate for the second adhesive layer, and a thermal laminate configuration using a thermoplastic material such as an acid-modified polyolefin-based resin for the second adhesive layer. The adhesive that is used in the dry laminate configuration has a highly hydrolysable bonding site such as an ester group and a urethane group, and thus a hydrolysis reaction due to hydrofluoric acid tends to occur. Therefore, a packaging material having the thermal laminate configuration is used in a use in which further higher reliability is demanded.
For example, the aluminum laminated lithium ion battery is formed as follows. A recessed portion is formed at part of the packaging material constituted by the multi-layer film by cold-molding in such a manner that the sealant layer is located on an inner side. A positive electrode, a separator, a negative electrode, and an electrolytic solution are put into the recessed portion. Then, the other portions of the packaging material are folded back and the same folded sealant layers at edge portions are heat sealed together for hermetic sealing. Recently, in order to efficiently accommodate an even larger amount of contents so as to increase an energy density, a lithium ion battery has been manufactured, in which the recessed portions are provided at both portions of the packaging material for a lithium ion battery, which are to be bonded to each other.
As a method of further increasing the energy density of the lithium ion battery, a method of increasing the contents accommodated in the recessed portion by making the recessed portion formed by the cold-molding even deeper may be exemplified. However, when the recessed portion is made deep, a pin hole or fracturing tends to occur at a side or a corner portion of the recessed portion during molding performed using a mold because the side or the corner portion is a portion in which a degree of stretching is particularly high.
As a packaging material in which moldability is improved, for example, a packaging material using a polyamide film, in which tensile strength or elongation up to fracturing in four directions (0°, 45°, 90°, and 135°) is controlled in a specific range, as the base material layer is known (refer to Japanese Patent No. 3567230). In addition, in order for the packaging material to resist deterioration even when the electrolytic solution adheres to an outermost surface during battery manufacturing, a packaging material using a laminated film, in which a polyethylene terephthalate film is laminated on an outer side of the polyamide film, as a base material layer is also known.
However, even when using the polyamide film in the base material layer as described above, it is difficult to say that durability after molding is sufficient. In addition, even when pin holes or fracturing does not occur during molding, peeling may occur between the base material layer and a metal foil layer after molding.